This invention relates to a method of making a ceramic insulator having a non-linear resistivity, stress-grading coating comprising particles of silicon carbide bonded with a glass.
It is proposed in U.S. Pat. No. 3,791,859-Hirayama to produce such insulators by providing a mixture of silicon carbide particles and glass particles, coating a ceramic body with this mixture, and firing the coated ceramic body. If the glass of this mixture is the usual glazing material for insulators, firing must be carried out at temperatures of 1850.degree. F or more. According to the Hirayama patent, firing at this high temperature can produce a reaction between the glass and the silicon carbide that causes bubbles or blisters to form which distort the surface of the insulator. To avoid this problem, Hirayama uses for his binder a low fusion-temperature glass having a fusion temperature of less than 850.degree. C (or 1562.degree. F). Such low fusion-temperature glass is more expensive than the usual glaze that is used for coating ceramic insulators and is not as durable as the usual glaze under adverse weather conditions. Moreover, the usual glaze has a long-standing proven record of compatibility with porcelain (e.g., in terms of relative thermal expansion coefficients contributing to increased mechanical strength of the overall insulator), which record the low fusion-temperature glasses do not have, insofar as I am aware.